Sunday, 3 April 2016

I bought some test leads with banana plugs and alligator clips for £0.99 on Ebay from Hong Kong. They were described as "Alligator Probe Test Leads Clip Pin to Banana Plug Cable for Digital Multimeter GF". I don't know what the GF means.

Very cheap and what could possibly be wrong with them? Well actually, almost everything!

The first time I used them to connect a regulator to a bench PSU they got hot and dropped several volts. Without needing to do any sums with wire gauge and current I felt the resistance must be too high, so I measured it to be about 1.4Ω for the round trip. Way too high as multimeter leads are generally about 0.2Ω.

I unscrewed a plug and found this: -

The screw bites down on the soft insulation and that presses the folded back strands against the barrel. Not the best way to make a connection as you want the screw biting down directly on the strands, or better still a ferrule.

At the clip end it was more of the same: -

The strands are trapped between metal and plastic again instead of being soldered through the hole. The crimp is there just for mechanical strain relief, not the electrical connection.

I removed all the connectors (the clips pull off really easily due to not being soldered) and measured the resistance of the wire on its own. Still 1.4Ω, too much I felt for that gauge of copper wire 2m long. The simple explanation is that it isn't copper.

The fact it sticks really well to magnets leads me to believe it is copper plated steel. That might be OK for measuring voltage but no good for measuring or carrying current. I can't see any reason for using it other than it must be cheaper.

I replaced it with 32/0.2mm copper wire half the length and got a total resistance of 20mΩ. Much more suitable for hooking up PSU test circuits but a bit less flexible than ideal for multimeter leads.

So basically I got usable connectors for £0.99, which is still probably cheaper than I could buy them for in the UK. The wire and the construction were junk.